Thermoplastic polyester elastomers exhibit excellent injection or extrusion moldability and high mechanical strength, and have applications in automobile parts, electric and electronic components, fibers, films, etc., as materials having excellent rubber properties (e.g., elastic recovery, impact resistance, and flexibility) and excellent cold resistance.
Thermoplastic polyester elastomers were considered unsuitable for blow molding due to their low melt viscosity. However, viscosity-increasing methods using a polyepoxy compound, a polyisocyanate compound, or an ionomer resin have become available. Taking advantage of the features, thermoplastic polyester elastomers have also been used in blow-molded products, such as flexible boots, for dust protection or grease retention of flexible drive couplings, such as constant-velocity joints of automobiles (e.g., PTL 1 to 3).
Due to demands, such as for light weight and a streamlined assembly of blow molded products, blow molding has also been required to address intricate shapes having both a thin portion and a thick portion. Typically, a relatively slow solidification rate of molten resin is advantageous in simplifying blow molding. However, a slow solidification rate in the thick portion is likely to cause defamation, such as sink marks. Thus, there has also been a demand for achieving the prevention of deformation of thick portions, such as sink marks, as well as excellent blow moldability.
The properties required for functional parts, such as flexible boots, include flexing fatigue resistance, heat aging resistance, oil resistance, and grease resistance. The requirements for the levels of such properties have been rising every year. In particular, grease resistance in a more severe environment, such as at a temperature exceeding 140° C., needs further improvement.